This expansion will include 70 new cards, 10 per faction plus 10 neutral cards, themed to connect each faction to a specific Witcher School with its own playstyle. Nilfgaard decks, for instance, will get the Viper School cards, which encourage "resource manipulation and sabotage". The Monster faction, being made of all those things witchers traditionally hunt, seems like it will be getting Way of the Mutant cards instead, which are all about "savagery and hunger".
Way of the Witcher ties in to the current Journey (Gwent's level-based progression systems), which has gone back to the days when the witcher mutation was first created and stars Alzur the Mage, who took most of the credit for it. Given that Gwent is The Witcher Card Game you'd think that witcher cards would already be featured heavily, but only a couple of them really see regular play—like Geralt of Rivia himself, who shows up in the Monsters deck that's currently at the top of the Gwent meta.
You can pre-order Way of the Witcher now.